1. Field
This invention relates to receiving method and apparatus, and more particularly to a method and apparatus for receiving radio frequency signals.
2. Description of Related Art
Conventional radio receiver circuits in which a received analog signal is downconverted in a first mixer stage to a first intermediate frequency, and subsequently downconverted in a second mixer stage to a second intermediate frequency are well known. It is also well known to sample the analog signal at the second intermediate frequency using an analog-to-digital converter.
Also well known are techniques of digital sub-sampling, whereby an analog-to-digital converter is used to achieve downconversion of a signal. These techniques rely upon the well-known phenomenon of signal “aliasing”. An analog-to-digital converter having a sampling rate (or sample frequency) of F can only entirely reliably reproduce signals having a frequency below (i.e., less than) F/2. Higher frequency signals are still detected, but these signals appear in the output digital signal at frequencies ranging from 0 to F/2. Thus, analog input signals having frequencies of f, (F−f), (F+f), (2F−f), (2F+f), etc. appear in an output signal at the frequency f.
The prior art digital sub-sampling techniques are utilized in a well-known manner to achieve downconversion of radio frequency signals. For example, one such prior art system is taught by Bella et al., in U.S. Pat. No. 5,630,227, issued on May 13, 1997. In particular, digital sub-sampling techniques can be used to downconvert a signal that only has components over a relatively narrow range of frequencies. For example, if an analog signal has frequency components only at one or more frequencies (designated (3F+f)) within a range from 3F to 3.5 F, and is sampled by an analog digital converter at a sampling frequency F, the output digital signal will have corresponding components at the frequency or frequencies f in the range from 0 to 0.5F. In other words, the frequency range from 3F to 3.5F is said to be “aliased” to a range from 0 to 0.5F.
Disadvantageously, the above-described well-known system is unable to effectively combat the detrimental effects of adjacent channel interference. Specifically, when a signal has a frequency that is relatively close to a frequency of one of the desired signals in the input, the above-described known system causes this signal to produce an output that interferes with the desired output signals in an unpredictable manner. In other words, the interferer (i.e., the signal that has a frequency that is relatively close to a frequency of one of the desired input signals) may alias to a frequency close to that at which a desired output signal will appear, and moreover may be a stronger signal than the desired signal, such that it cannot easily be removed through filtering.
Therefore, a need exists for a method and apparatus for receiving radio frequency signals in a communication system that can be easily implemented and overcomes the disadvantages of other methods and apparatuses such as the above-described known systems. The present disclosure provides such a radio frequency receiver method and apparatus.